1. Field of the Invention
The present invention relates to a flame-retardant polyamide resin composition. More particularly, the present invention relates to a flame-retardant polyamide resin composition adequate for producing shaped articles having excellent flame resistance and other physical and chemical properties and having a satisfactory appearance.
2. Description of the Prior Art
It is known from, for example, Japanese Examined Patent Publication (Kokoku) No. 47-41745 (1972) and Japanese Unexamined Patent Publication (Kokai) No. 51-39750 (1976) that a polyamide resin can be blended with a flame retarder consisting of melamine, cyanuric acid, and/or isocyanuric acid to impart a flame-retarding property to the polyamide resin. It is also known that the flame-retarding effect of cyanuric acid and isocyanuric acid is superior to that of melamine.
Polyamide resin compositions containing the above-mentioned conventional flame retarders are, however, disadvantageous in that when the compositions are molded, the flame retarder sublimes at the molding temperature and soils the mold. Also, if the resultant shaped article is exposed to a moisture-containing atmosphere or is stored for a long period of time, a portion of the flame retarder moves from the inside to the outer surface of the shaped article to form a thin layer of fine flame retarder particles degrading the appearance of the shaped article. This latter phenomenon is referred to as bleeding out.
When cyanuric acid and/or isocyanuric acid is used as a flame retarder, the resultant shaped article has the further disadvantage, that the cyanuric and/or isocyanuric acid decomposes during the shaping process and generates bubbles distributed in a portion of the shaped article.
In order to eliminate the above-mentioned disadvantages of conventional flame retarders containing melamine, cyanuric acid, and/or isocyanuric acid, it has been attempted to use, as a flame retarder, an equimolar reaction product of melamine with cyanuric acid, that is, melamine cyanurate. This attempt is disclosed in U.S. Pat. No. 4,298,518. The flame retarder comprising melamine cyanurate is considerably effective for decreasing the disadvantages of the flame retarders containing melamine, cyanuric acid, and/or isocyanuric acid. However, this type of flame-retarder comprising melamine cyanurate is still not satisfactory when used for producing part members of electrical machinery and apparatuses requiring a very high flame-retarding property.
According to the experience of the present inventors, the flame-retarding effect of the melamine cyanurate was deemed to be poorer than that of melamine and cyanuric acid. For example, a composition consisting of 80% by weight of a nylon 6 resin and 20% by weight of melamine can meet the flame-retarding level V-0 in accordance with UL-94 (Underwriters Laboratories Inc. of U.S.A.) Standard. In order to meet the flame-retarding level V-0, it is necessary that five test specimens satisfy all of three items, that is, (1) an average combustion time of the specimens must be 5 seconds or less, (2) a longest combustion time must be 10 seconds or less, and (3) in all of combustion tests repeated for five specimens, when an absorbent cotton wadding mass is placed 12 inches below the lower ends of the specimens, no cotton wadding mass may be burnt by molten drops from the specimens.
In the case of a composition consisting of 80% by weight of a nylon 6 resin and 20% by weight of melamine cyanurate, the test specimens prepared from the composition can satisfy only items (1) and (2) and not item (3). That is, when the five specimens are subjected in sequence to the combustion test (V-0 level), one or two specimens always suffer from burning of the absorbent cotton mass by molten drops. That is, the composition consisting of the polyamide resin and melamine cyanurate cannot meet the flame-retarding level V-0 of the UL-94 Standard.
In another attempt for eliminating the disadvantages of melamine, cyanuric acid, and isocyanuric acid, Japanese Unexamined Patent Publication (Kokai) Nos. 53-47451 (1978) and 56-22347 (1981) disclose flame-retardant polyamide resin compositions containing a melamine derivative, for example, methylene dimelamine or ethylene dimelamine, as a flame retarder. Studies by the present inventors showed that although the flame retarder comprising the above-mentioned melamine derivative exhibited a superior effect in imparting flame resistance to the polyamide resin compared to melamine cyanurate, the flame-retarding property of the resultant polyamide composition containing about 10% by weight or less of the melamine derivative is not always satisfactory and, therefore, should be improved.
Under the above-mentioned circumstances, there is great demand in the polyamide resin industry for a new type of flame-retardant polyamide resin composition which exhibits an excellent flame retarding property and is free from the disadvantages of the conventional compositions.